The invention relates to a process and an apparatus for producing a plastic blow-molded part.
In the production of blow-molded parts, a parison is in most cases negatively preformed for forming the contours. This has the effect that, when there is great stretching, thin points occur in critical regions and may lead to the failure of the component. Although this occurrence can be counteracted by a greater initial wall thickness, this leads to higher material consumption and greater wall thickness variations and to higher component costs overall.
A further disadvantage in the production of blow-molded parts with great stretching is that the surface quality is impaired when producing parts with inserted decorative materials (for example textile, film etc.) on account of the great distortion.
It is accordingly an object of the invention to provide a process and an apparatus for producing a plastic blow-molded part that overcomes the above-mentioned disadvantages of the prior art methods and devices of this general type, in which stretchings that are as uniform as possible can be achieved in the parison and the greatest possible remaining wall thickness in comparison with the initial parison can be achieved in the critical regions, it being intended for the use of material to be minimized and the drawing-out behavior and surface quality to be favorably influenced.
With the foregoing and other objects in view there is provided, in accordance with the invention, a process for producing a plastic blow-molded part. The process starts by providing a plastically deformable plastic parison having a first subregion and an opposite second subregion. The parison is placed between a molding element having at least one projecting region and a mold interacting with the molding element. An edge region of the parison is held in a fixed position which is adapted to a mold parting region of the plastic blow-molded part to be produced resulting in a fixedly held edge region. A positive pressure is generated between the molding element and the first subregion and within the parison for keeping the first subregion at a distance from the molding element and to keep the first subregion and the second subregion at a distance from each other. At least one stretching region of the first subregion is preformed by moving the molding element in relation to the fixedly held edge region in a direction of the plastic blow-molded part and by generating a negative pressure between the projecting region and the stretching region of the first subregion to bring at least the stretching region of the first subregion to bear against the projecting region of the molding element. The first subregion and the second subregion are deformed for generating the positive pressure within the parison, the first subregion being brought to bear against the molding element and the second subregion being brought to bear against the mold.
It is preferably that the first subregion is preformed by generating a negative pressure between the molding element and the entire first subregion, in order to bring the first subregion in its entirety to bear against the molding element.
It may be provided that a negative pressure is generated between the mold and the second subregion, in order to bring the second subregion to bear against the mold.
In a preferred embodiment it is provided that the mold is itself opened by moving two or more mold elements apart and, is closed by moving the mold elements together to a contour of the blow-molded part to be produced. In this case, it may be provided that a negative pressure is generated between the mold or its mold elements and the second subregion.
In one embodiment it may be provided that, a decorative material is introduced on a visible side of the blow-molded part to be produced.
The plastic parison may be a coextruded parison having at least two layers. The parison may in this case have at least one reinforced, preferably fiber-reinforced, layer and an unreinforced layer.
The molding element may be formed by a (first) mold that interacts with the (second) mold. Alternatively, it may be provided that the molding element is formed by a plastic molded part which interacts with the (second) mold. In this case, the plastic molded part is preferably covered with a decorative material.
With the foregoing and other objects in view there is further provided, in accordance with the invention, an apparatus for producing a plastic blow-molded part. The apparatus is formed of a molding element having at least one projecting region with openings formed therein which can be connected to a negative pressure source. A molding box surrounds the molding element and can be subjected to negative pressure. The molding box and the molding element are movable in relation to each other and the molding box has a peripheral edge for one of securely holding and securely clamping an edge region of a parison. The apparatus also has a mold disposed opposite the molding element.
It is preferably provided that the contour elements for preforming the parison are disposed in the molding box.
It is preferably provided that the mold is of a divided configuration, mold elements of the mold being movable in such a way that, in the moved-together state, they can be brought to bear against a contour of the molded part to be produced, in particular against the one projecting region of the molding element. It is at the same time expediently provided that the mold elements of the mold are provided with openings that can be connected to a negative pressure source.
In a preferred embodiment, it is provided that the preforming mold is formed at least in certain regions as a jointed band with a number of articulation points.
It may be provided that the molding element is configured as a (first) mold which interacts with the (second) mold. Alternatively, the molding element may be a preformed plastic part that interacts with the mold.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a process and an apparatus for producing a plastic blow-molded part, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.